1. Field of the Invention
The present invention relates to a display apparatus including a display panel in which a variety of display elements, which are light emitting elements such as organic electroluminescent (EL) elements, are driven to emit light, and more particularly to a display apparatus using a precharge method in which display elements are precharged with an electrical charge before being driven to emit light, thereby increasing the response speed.
2. Description of the Related Art
Examples of a display apparatus including organic electroluminescent (EL) elements that are driven to emit light are described in Japanese Unexamined Patent Publication Kokai Nos. 9-232074 (see, for example, FIGS. 9 to 12) and 2005-18038 (see, for example, FIG. 1).
FIG. 5 schematically illustrates a conventional organic EL display, for example, described in the above-described Japanese unexamined patent publications.
The organic EL display shown in FIG. 5 is a passive matrix organic EL display, which employs a cathode line scan/anode line drive method, and includes a display panel 10, an anode line drive circuit 20 which drives anode lines, a cathode line scan circuit 30 which scans cathode lines, and a display control circuit 40 which controls the anode line drive circuit 20 and the cathode line scan circuit 30.
The display panel 10 includes a plurality of anode lines 11-1 to 11-N and a plurality of cathode lines 12-1 to 12-M, which are arranged in matrix form, and a plurality of display elements 13-11 to 13-MN, each including an organic EL element, which are arranged respectively at intersections between the anode lines 11-1 to 11-N and the cathode lines 12-1 to 12-M and each of which is connected between a corresponding one of the anode lines 11-1 to 11-N and a corresponding one of the cathode lines 12-1 to 12-M. Each of the display elements 13-11 to 13-MN including an organic EL element which is a current injection type light emitting diode wherein electrons and holes are injected through corresponding electrodes and are then recombined in an organic material, thereby emitting light. Each of the display elements 13-11 to 13-MN includes an organic light emitter and has a capacitance between the anode and cathode.
The anode line drive circuit 20, which drives the anode lines 11-1 to 11-N, is connected to the anode lines 11-1 to 11-N. The anode line drive circuit 20 includes a plurality of series circuits and a plurality of precharge switches 23-1 to 23-N which are connected in parallel respectively with the plurality of series circuits. The plurality of series circuits includes constant current circuits 21-1 to 21-N and output drivers 22-1 to 22-N connected in series between a power supply voltage (VDDH) node and the anode lines 11-1 to 11-N, respectively. The constant current circuits 21-1 to 21-N are turned on/off according to drive control signals C1 to CN having specific pulse widths and output constant currents when they are on. The precharge switches 23-1 to 23-N are turned on/off according to precharge control signals P1 to PN having specific pulse widths. The precharge switches 23-1 to 23-N allow the display elements 13-11 to 13-MN to be precharged by the power supply voltage VDDH when they are on. When a constant current or voltage is applied to the display elements 13-11 to 13-MN, each including an organic EL element, the luminance of the display elements slowly rises (i.e., increases at a low rate) to a target level since the display element has capacitance. Particularly when pulse width modulation (PWM) control is performed, the capacitance of the display elements 13-11 to 13-MN causes inaccurate pulse widths to be applied to the display elements. In actual driving of the display elements 13-11 to 13-MN, the slow rise in the luminance is avoided by previously charging the display elements 13-11 to 13-MN, each having capacitance, with an electrical charge received through the precharge switches 23-1 to 23-N, i.e., by charging the display elements 13-11 to 13-MN to a voltage lower than a light emitting threshold voltage.
The cathode line scan circuit 30, which sequentially scans the plurality of cathode lines 12-1 to 12-M, is connected to the plurality of cathode lines 12-1 to 12-M. The cathode line scan circuit 30 includes a plurality of scan switches 31-1 to 31-M connected respectively to the cathode lines 12-1 to 12-M. The scan switches 31-1 to 31-M are sequentially switched according to scan control signals R1 to RM having specific pulse widths. To cause corresponding ones of the display elements 13-11 to 13-MN to emit light, each of the scan switches 31-1 to 31-M is switched to a ground potential node VSSH to connect a corresponding one of the cathode lines 12-1 to 12-M to the ground potential node VSSH. To prevent corresponding ones of the display elements 13-11 to 13-MN from emitting light, each of the scan switches 31-1 to 31-M is switched to the power supply voltage node VDDH to connect a corresponding one of the cathode lines 12-1 to 12-M to the power supply voltage node VDDH.
The display control circuit 40 outputs control signals C1 to CN, P1 to PN, and R1 to RM for controlling the anode line drive circuit 20 and the cathode line scan circuit 30. The display control circuit 40 includes a shift register 41, a display data latch circuit 42, and a driver control unit 43. According to a clock signal CLK, the shift register 41 receives serial display data DA, which determines gray or luminance gradation levels for display, and converts the received serial display data to parallel display data and outputs it to the display data latch circuit 42. The display data latch circuit 42 stores the parallel display data output from the shift register 41 and outputs the stored parallel display data to the driver control unit 43. The driver control unit 43 outputs control signals C1 to CN, P1 to PN, and R1 to RM at specific times based on the parallel display data output from the display data latch circuit 42.
FIGS. 6A and 6B are graphs each illustrating a driving waveform for driving a display element (for example, 13-22). FIG. 6A illustrates a waveform when driving the display element at a low brightness level or gray scale (for example, black). FIG. 6B illustrates a waveform when driving the display element at a high brightness level (for example, white). A description will now be given of how the display element 13-22 is driven to emit light.
When the time to change the scan target to the cathode line 12-2 is reached while the cathode line 12-1 is scanned with the scan switch 31-1 switched to the ground potential VSSH according to the control signal R1, first, the scan switch 31-2 is switched to the ground potential VSSH according to the control signal R2 and the precharge switch 23-2 is turned on according to the control signal P2 at the same time. This causes a power supply current to flow via a route (power supply voltage VDDH->precharge switch 23-2->anode line 11-2->display element 13-22->cathode line 12-2->scan switch 31-2->ground VSSH), thereby precharging the display element 13-22.
Then, the precharge switch 23-2 is turned off according to the control signal P2 and the constant current circuit 21-2 is turned on according to the control signal C2. The control signal C2 has a pulse width corresponding to display data DA. When the display element 13-22 is driven to emit light of the lowest gray level (i.e., black) as shown in FIG. 6A, the pulse width of the control signal C2 is zero, so that the constant current circuit 21-2 is actually turned off rather than turned on. Accordingly, the display element 13-22 is activated by the ground potential VSSH. Specifically, an electrical charge, with which the display element 13-22 is precharged, is discharged to the ground potential node VSSH via the scan switch 31-2, so that the display element 13-22 is prevented from emitting light, thus displaying black.
On the other hand, when the display element 13-22 is driven to emit light of a high gray level (for example, white) as shown in FIG. 6B, the pulse width of the control signal C2 is large, so that the constant current circuit 21-2 outputs a constant current according to the power supply voltage VDDH during a period of time corresponding to the pulse width and the output driver 23-1 drives and provides the constant current to the display element 13-22. This causes the display element 13-22 to emit light, thus displaying white. When the pulse width period is completed, the constant current circuit 21-2 is turned off and an electrical charge stored in the display element 13-22 is discharged to the ground potential node VSSH via the scan switch 31-2, so that the display element 13-22 is prevented from emitting light, thus displaying black.
However, the conventional display has the following problems. To ensure that the luminance of the display elements 13-11 to 13-MN rapidly rises, they are precharged to the power supply voltage VDDH before they are driven by the constant current circuits 21-1 to 21-N and the output drivers 22-1 to 22-N. The display elements 13-11 to 13-MN, each having a capacitance, are precharged with an electrical charge, regardless of the gray level for display. When each of the display elements 13-11 to 13-MN is driven to display data of a low gray level, an electrical charge, with which the display element has been precharged, is not immediately removed from the display element, thus failing to display a clean low gray level.
More specifically, in FIGS. 6A and 6B, first, the precharge switch 23-2 is turned on to precharge the display element 13-22 to the power supply voltage VDDH. Even after the precharge switch 23-2 is turned off, an electrical charge remains in the display element 13-22 to apply the power supply voltage VDDH to the display element 13-22 during the period in which the drive circuit including the constant current circuit 21-2 and the output driver 22-2 is turned on. In the case where the display element 13-22 is driven to emit light at a high gray level as shown in FIG. 6B, the voltage applied to the display element 13-22 drops to the ground potential VSSH after it is driven by the power supply voltage VDDH for a certain period of time. On the other hand, in the case where the display element 13-22 is driven to emit light at a gray level of “0” as shown in FIG. 6A, it is necessary that the voltage applied to the display element 13-22 immediately drop to the ground potential VSSH. However, when it is driven to emit light at a gray level of “0” as shown in FIG. 6A, it takes a certain time to discharge the display element 13-22 to the ground potential VSSH since it has been precharged by the power supply voltage VDDH. Until the display element 13-22 is completely discharged to the ground potential VSSH, a voltage is still applied to the display element 13-22, so that it emits a glimmer of light, thus failing to display a clean gray level “0”.
In addition, the superfluous precharging leads to unnecessary current consumption.